In the production of miniaturized objects, e.g., miniature devices e.g., integrated circuits and microelectronics for semiconductor and display applications, the tools and auxiliary structures used in their manufacture, as well as the miniaturized objects themselves have to be examined carefully. Optical methods of examining these tools, objects and structures are non-destructive and frequently preferred over other approaches. Hence, advances in optical examination of miniature patterns or features are important.
In many cases miniaturized devices are made by photolithographic techniques. In a typical application of the photolithographic technique, a layer of photoresist is deposited on a substrate or other device layer and then exposed to radiation of appropriate wavelength through a patterning mask. Certain regions of the photoresist layer are exposed, and other are not, according to the pattern defined in the mask. Exposing the photoresist to radiation changes its solubility. After exposure, solvent is used to remove regions of higher solubility photoresist, leaving regions of "hardened" photoresist at sites on the device layer as dictated by the patterning mask. The "hardened" photoresist remains to protect the underlying material from removal during a subsequent etching step or other suitable material removal procedure. After etching the photoresist is discarded. In this manner, a feature is created in the device based on the pattern defined in the mask.
Clearly, the photoresist layer must be accurately patterned to form features to the exacting specifications for miniature devices. It is therefore desirable to monitor the photolithographic process at various stages and on a periodic basis. For example, it would be desirable to measure the thickness of the photoresist layer and examine the pattern to determine feature sizes. This may be done by subjecting the photoresist to ultraviolet light having a wavelength in the range of 300 to 800 nm and measuring the reflected radiation. The reflected radiation may be correlated to photoresist thickness. The general principle of this measurement technique is that the measured light reflected from a substrate is modulated by constructive and destructive optical interference from an overlying semitransparent material such as photoresist. For more information see Chopra, K. L., Thin Film Phenomena, p. 99 (McGraw Hill, 1969). The periodicity of the reflectance spectra can also be used to determine optical properties, such as the refractive index n of the substrate.
Measurement of the pattern or features is a more difficult procedure. For example, in a typical application, the pattern consists of a plurality of stripes and spaces, e.g. a line and space pattern. These types of patterns are frequently encountered in forming diffractive elements such as lenses or gratings in semiconductors or glass, forming fluid flow microchannels in silicon, and in general for providing a variety of mechanical features in a substrate. In measuring stripe widths and separations the prior art techniques have typically relied on scanning electron microscopy (SEM).
The prior art also offers interferometric techniques for measuring repeating patterns. These can be used to examine highly regular patterns such as gratings.
More recently, attempts have been made to measure patterns using scatterometry. In this technique, a pattern is subjected to light, such as from a laser, typically having a single wavelength. The light is usually directed toward the pattern at some angle to the normal. The light reflected from the pattern is reflected at various orders, i.e., angles relative to the incident light. The amount or intensity of light reflected at various orders is measured. It may be possible to use such data to obtain quantitative information about the pattern. However, scatterometry is very sensitive to changes in the profile of the pattern, i.e., the height of lines, and requires relatively sophisticated correlation work to relate the reflected radiation to the features of a pattern. Other examples of characterization methods pertaining to photolithography and equipment suitable for practicing such methods are described in U.S. Pat. Nos. 5,363,171; 5,184,021; 4,866,782 and 4,757,207.
Another approach to measuring micro-sized patterns is discussed in U.S. Pat. No. 5,607,800 to Ziger. This method and arrangement for characterizing features of a patterned material on an underlayer is based on selecting an appropriate wavelength range where the patterned material absorbs more radiation than the underlayer. In other words, substrate or underlayer is more reflective than the pattern or surface features in this wavelength range. The reflectance spectrum uniquely identifies the pattern and can be used to study similar patterns by comparing their reflectance spectra. Unfortunately, just as in the case of scatterometry when patterns vary this comparison-based approach can not be used effectively to study altered patterns.
In fact, all of the above approaches to optically measuring miniature patterns or features are limited in their applicability. What is needed is a more versatile approach to examining miniature patterns with varying feature sizes.